The object of the proposed study is to use the mouse as a tool to investigate four distinct areas of cytogenetics: 1. Linkage homologies among mammals; 2. Inheritance of chromosomal imbalance from translocation carriers; 3. Mechanism of X-inactivation of autosomal loci in X-autosomal translocations; and 4. Relationship of gene-dosage to gene-product. Methods consist of recombination determination and recombinant inbred strain analysis; syntene mapping in somatic cell hybrids, duplication-deficiency mapping using chromosomally unbalanced embryos, analysis of autosomal loci coding for isozyme variants and DNA replication kinetics in X-autosomal translocation heterozygotes, and determinations of gene products (enzymes) in individuals with defined variations in dosages for the structural gene locus in question.